The Field Reversed Configuration (FRC) belongs to the class of magnetic plasma confinement topologies known as compact toroids. It exhibits predominantly poloidal magnetic fields and possesses zero or small self-generated toroidal fields (see M. Tuszewski, Nucl. Fusion 28, 2033 (1988)). The attractions of such a configuration are its simple geometry for ease of construction and maintenance, a natural unrestricted divertor for facilitating energy extraction and ash removal, and very high average (or external) β (β is the ratio of the average plasma pressure to the average magnetic field pressure inside the FRC), i.e., high power density. Theft metric is also a very good measure of magnetic efficiency. A high average β value, e.g. close to 1, represents efficient use of the deployed magnetic energy and is henceforth essential for the most economic operation. High average β is also critically enabling the use of aneutronic fuels such as D-He3 and p-B11.
The traditional method of forming an FRC uses the field-reversed θ-pinch technology, producing hot, high-density plasmas (see A. L. Hoffman and J. T. Slough, Nucl. Fusion 33, 27 (1993)). A variation on this is the translation-trapping method in which the plasma created in a theta-pinch “source” is more-or-less immediately ejected out of the formation region and into a confinement chamber. The translating plasmoid is then trapped between two strong mirrors at the ends of the confinement chamber (see, for instance, H. Himura, S. Okada, S. Sugimoto, and S. Goto, Phys. Plasmas 2, 191 (1995)). Once in the confinement chamber, various heating and current drive methods may be applied such as beam injection (neutral or neutralized), rotating magnetic fields, RF or ohmic heating, etc. This separation of source and confinement functions offers key engineering advantages for potential future fusion reactors. FRCs have proved to be extremely robust, resilient to dynamic formation, translation, and violent capture events. Moreover, they show a tendency to assume a preferred plasma state (see e.g. H. Y. Guo, A. L. Hoffman, K. E. Miller, and L. C. Steinhauer, Phys. Rev. Lett. 92, 245001 (2004)). Significant progress has been made in the last decade developing other FRC formation methods: merging spheromaks with oppositely-directed helicities (see e.g. Y. Ono, M. Inomoto, Y. Ueda, T. Matsuyama, and T. Okazaki, Nucl. Fusion 39, 2001 (1999)) and by driving current with rotating magnetic fields (RMF) (see e.g. I. R. Jones, Phys. Plasmas 6, 1950 (1999)), which also provides additional stability.
FRCs consist of a torus of closed field lines inside a separatrix, and of an annular edge layer on the open field lines just outside the separatrix. The edge layer coalesces into jets beyond the FRC length, providing a natural divertor. The FRC topology coincides with that of a Field-Reversed-Mirror plasma. However, a significant difference is that the FRC plasma can have an internal β of about 10. The inherent low internal magnetic field provides for a certain indigenous kinetic particle population, i.e. particles with large larmor radii, comparable to the FRC minor radius. It is these strong kinetic effects that appear to at least partially contribute to the gross stability of past and present FRCs, such as those produced in the recent collision-merging experiments.
The collision-merging technique, proposed long ago (see e.g. D. R. Wells, Phys. Fluids 9, 1010 (1966)) has been significantly developed further: two separate theta-pinches at opposite ends of a confinement chamber simultaneously generate two plasmoids (e.g., two compact tori) and accelerate the plasmoids toward each other at high speed; they then collide at the center of the confinement chamber and merge to form a compound FRC. In the construction and successful operation of one of the largest FRC experiments to date, the conventional collision-merging method was shown to produce stable, long-lived, high-flux, high temperature FRCs (see e.g. M. Binderbauer, H. Y. Guo, M. Tuszewski et al., Phys. Rev. Lett. 105, 045003 (2010), which is incorporated herein by reference). In a related experiment, the same team of researchers combined the collision-merging technique with simultaneous axial acceleration and radial compression to produce a high density transient plasma in a central compression chamber (see V. Bystritskii, M. Anderson, M. Binderbauer et al., Paper P1-1, IEEE PPPS 2013, San Francisco, Calif. (hereinafter “Bystritskii”), which is incorporated herein by reference). This latter experiment reported in Bystritskii utilized a multitude of acceleration and compression stages before final collisional merging and represents a precursor concept to the system subject to this patent application.
In contrast to the embodiments described here, the precursor system described in Bystritskii featured simultaneous compression and acceleration of compact tori within the same stage by using active fast magnetic coils. Five such stages were deployed on either side of a central compression chamber before magnetically compressing the merged compact tori. While the precursor experiment achieved respectable performance, it exhibited the following deficiencies: (1) Simultaneous compression and acceleration led to inefficient use of driver energy deployed for magnetic compression due to a timing mismatch; (2) Temperature and density decreased as plasma expanded during transit between sections; (3) Abrupt transitions between adjacent sections led to large losses due to plasma-wall contact and generation of shockwaves.
Aside from the fundamental challenge of stability, pulsed fusion concepts in the medium density regime will have to address adequate transport timescales, efficient drivers, rep-rate capability and appropriate final target conditions. While the precursor system has successfully achieved stable single discharges at encouraging target conditions, the collective losses between formation and final target parameters (presently about 90% of the energy, flux, and particles) as well as the coupling efficiency between driver and plasma (at present around 10-15%) need to be substantially improved.
In light of the foregoing, it is, therefore, desirable to provide improved systems and methods for pulsed fusion concepts that facilitate a significant reduction of translation and compression losses and an increase in driver efficiency.